This invention relates to a method for making an etch-resistant stencil on a metal work piece, particularly a metal sheet. The novel method may be used, for example, in a method for etching an apertured mask for use in a color television picture tube.
The preparation of apertured masks by photoexposure and etching has been described previously; for example, in U.S. Pat. Nos. 2,750,524 to F. G. Braham, 3,745,011 to D. E. Hudgin, 3,751,250 to J. J. Moscony et al, 3,834,905 to J. A. Dodd, Jr. et al, and 3,929,532 to H. W. Kuzminski. In a typical process, light-sensitive coatings are applied to both major surfaces of a thin metal sheet, such as of cold-rolled steel or a copper-nickel alloy. The coatings are exposed to light images, as by contact-printing exposure, to render the exposed portions thereof less soluble in a particular solvent. The exposed coatings are developed to remove the more-soluble unexposed portions thereof thereby producing a stencil on each surface of the sheet, and then baked to render the retained, less-soluble, exposed portions etch resistant. Then, the sheet with the etch-resistant stencil thereon is selectively etched as desired, after which the stencils are removed from the sheet.
In some prior methods, the light-sensitive coatings are comprised of dichromate-sensitized protein material, such as fish glue, albumin, and/or casein. The coatings are prepared from a liquid coating composition comprising the protein material, alkali-dichromate photosensitizer therefor and water. A layer of this aqueous coating composition is applied to the metal surface or surfaces and then dried to produce the light-sensitive coating. Where casein has been used, the liquid coating composition includes an alkalizing agent, which is an alkali compound such as NH.sub.4 OH, NaOH, Na.sub.2 HPO.sub.4 and Na.sub.2 CO.sub.3, in such concentration as to impart a pH of more than 7.0 to the coating composition. An alkalizing agent is added in order to render the casein therein sufficiently soluble to deposit an adequate amount of casein to produce an etch-resistant stencil. With the addition of one of the usual alkalizing agents, the light-sensitive coating is poorly adherent to the surface to be etched, and the etch-resistant stencil is weak and soft. Also, coating compositions which have pH values above 7.5 usually produce coatings with lower photosensitivity and therefore require longer exposures for producing the stencil.